1. Field of the Invention
The present invention relates to a focusing and tracking system for an optical storage apparatus which stores information on the surface of an optical disk in the form of a large number of pits aligned in tracks and, more particularly, to a servo system for focusing and tracking an optical beam, such as a laser beam, on the surface of the optical disk for reading information stored on the disk by processing the optical beam after it is reflected by the disk.
2. Description of the Related Art
Optical storage devices generally store binary signal information in the form of a large number of pits formed on the surface of an optical disk. The information is readable by scanning the pits, which are aligned in circular tracks or on a spiral track, with an optical beam (hereinafter a laser beam). The laser beam is focused to form a small spot having a diameter of approximately 1 .mu.m on the surface of the disk and guided to follow the tracks on the disk. Information is read from the disk by detecting the laser beam reflected by the surface of the disk and modulated by the pits formed thereon. Modulations of the laser beam include, for example, diffraction modulation, absorption modulation, and reflection modulation, depending on the type of optical storage apparatus.
The pitch of the tracks is small, typically 1.6 .mu.m, and the size of the pits aligned on the tracks is on the order of 0.8 to 1.0 .mu.m. Thus, an optical storage medium achieves an extraordinarily high signal density in comparison with the signal density of a magnetic storage medium. In order to accurately read information from an optical disk, a highly accurate servo system is necessary to center the laser beam on the tracks. The tracks, however, may be eccentric, due to insufficient dimensional preciseness of the disk, having positional variations on the order of 100 .mu.m. This results in cyclic variations of the positions of the tracks in the radial direction when the disk is rotated, causing difficulties in achieving accurate tracking. In addition, there are difficulties in focusing the optical beam, since an objective lens with a high numerical aperture, such as 0.5, is used in the optical storage apparatus in order to permit the reading of extremely fine details, resulting in a very small depth of field, on the order of several .mu.m. Further, the disk surface may have variations in the vertical direction on the order of 100 .mu.m due to distortions of the disk.
In spite of the above-mentioned conditions, the laser beam must be centered on a track with an accuracy of approximately 0.1 .mu.m to avoid cross-talk, and the laser beam must be focused on the surface with an accuracy of approximately 0.5 .mu.m. Thus, an accurate servo system for centering and focusing the laser beam on the tracks of the optical disk is a key feature of an optical storage apparatus. Various servo systems for focusing and tracking an optical beam have been developed and are reported in references such as "Optical Read Out of Video Disks," by C. Bricot et al., IEEE Transactions C.E., Nov., 1976, p. 304.
In most focusing servo systems, an objective lens for focusing an optical beam is moved by an electrodynamic coil similar to those utilized in acoustic loud speakers. The signals for driving the electrodynamic coil, namely error signals, may be obtained by using optical sensors which detect the laser beam reflected form the information carrying surface. Typical optical systems used in practice are astigmatic and asymmetric systems.
In an astigmatic sensor, error signals are derived by an optical system having a cylindrical lens placed in the path of the reflected laser beam and four photosensors connected in pairs to differential amplifiers. With such an astigmatic system, the number of optical elements in a system is rather large and the system is costly.
In an asymmetric sensor, a mask edge is placed in the path of the reflected laser beam to shade part of the laser beam, and two photosensors connected to a differential amplifier provide error signals. The asymmetric sensor, therefore, is less complicated than an astigmatic sensor. Both astigmatic and asymmetric systems, however, include optical elements which must be assembled with an extremely high degree of dimensional accuracy in the direction parallel to the surface of the disk as well as the direction perpendicular to the disk, namely, in three dimensions, adversely affecting the cost and quality of the apparatus.
For radial tracking servo systems, there are several methods for generating error signals, the particular method being selected in accordance with the type of modulation of the reflected laser beam. Typically, radial tracking is accomplished by reflecting the laser beam using a tilting mirror which is driven by an electrodynamic activator in accordance with tracking error signals. One example of a conventional system for obtaining an error signal will be described for use with optical disks designed for diffraction modulation, namely, disks having pregrooves. In this method, a single laser beam is used to obtain error signals for driving a tilting mirror. When the laser spot is not well centered on the track, the reflected beam has a asymmetrical intensity pattern. The intensity pattern is detected by two photosensors connected to a differential amplifier. This method is referred to as a "push-pull" method; however, this method is not applicable to flat optical disks, i.e., disks which do not have pregrooves.
For optical disks having a flat surface and providing reflection modulation, a tracking servo system detects an asymmetric distribution which occurs when the beam is not well centered on the track, with two or more photosensors.
Simplification of the tracking servo system is necessary to improve the performance, quality and reliability of optical storage apparatus. Furthermore, there are problems inherent to optical systems which provide disturbances caused by small but inevitable optical defects, referred to as "exterior disturbances".